枚举概述
枚举用来管理一组相关常量的集合，使用枚举可以提高程序的可读性，使代码更清晰且更易于维护。
在 Java 5 之前没有提供枚举类型，可以通过声明静态常量（final static 变量）替代枚举常量。
通常在接口中声明一组静态常量，当然也可以在一般类中声明一组静态常量。这些常量往往都是 int 类型，这是为了以后方便使用 switch 语句进行判断。
public interface WeekDays {
// 枚举常量列表
int MONDAY = 0; // 星期一
int TUESDAY = 1; // 星期二
int WEDNESDAY = 2; // 星期三
... ...
}
使用常量的缺点：
- 类型不安全。程序执行过程中很有可能给 day 变量传入一个任意的整数值，可能导致程序出现错误。
- 程序不方便调试。在程序调试时，如果通过日志输出day值，那么只能看到 0~4 之间的数值。
枚举类型可以避免直接使用常量所导致的问题。Java 5 之后可以使用枚举类型了，Java中枚举类型的作用已经不仅仅是定义一组常量提高程序的可读性了，还具有如下特性：
- Java 枚举类型是一种类，是引用类型，具有了面向对象特性，可以添加方法和成员变量等；
- Java 枚举类型继承 java.lang.Enum，且不需要显式声明；
- Java 枚举类型可以实现接口，与类实现接口类似；
- Java 枚举类型不能被继承，不存在子类；
- 枚举类型都实现了Java API中的 Comparable 接口，都可以通过方法compareTo与其他枚举值进行比较。比较其实就是比较 ordinal 的大小；
- 可搭配 switch 语句使用，在 switch 语句内部，枚举值不能带枚举类型前缀。
枚举类声明
[public] enum 枚举名 {
枚举常量列表
}
“枚举名”是该枚举类的名称。它首先应该是有效的标识符，其次应该遵守 Java 命名规范。建议以 Enum 结尾。
枚举类中成员变量和成员方法
枚举类可以像类一样包含成员变量和成员方法，成员变量可以是实例变量也可以是静态变量，成员方法可以是实例方法，也可以是静态方法，但不能是抽象方法。
枚举类构造方法
public enum WeekDayEnum {
// 枚举常量列表
MONDAY("星期一", 0), TUESDAY("星期二", 1), WEDNESDAY("星期三", 2), THURSDAY(
"星期四", 3), FRIDAY("星期五", 4);
// 实例变量
private final String name;
private final int index;
private WeekDayEnum(String name, int index) {
this.name = name;
this.index = index;
}
}
注意枚举类的中的构造方法只能是私有访问级别，构造方法可以省略 private 关键字，但它仍然是私有的构造方法。这也说明了枚举类不允许在外部创建对象。
枚举常用方法
所有枚举类都继承java.lang.Enum类，Enum中定义了一些枚举中常用的方法：
-
int ordinal()：返回枚举常量的顺序。每个枚举值经常有一个关联的标识符，为枚举常量声明的顺序，ordinal值会从从数字 0 开始。 -
枚举类型[] values()：静态方法，返回一个包含全部枚举常量的数组。 -
枚举类型 valueOf(String str)：静态方法，str 是枚举常量对应的字符串，返回一个包含枚举类型实例。
jdk1.8 Enum 的源码
public abstract class Enum<E extends Enum<E>>
implements Comparable<E>, Serializable {
/**
* The name of this enum constant, as declared in the enum declaration.
* Most programmers should use the {@link #toString} method rather than
* accessing this field.
*/
private final String name;
/**
* Returns the name of this enum constant, exactly as declared in its
* enum declaration.
*
* <b>Most programmers should use the {@link #toString} method in
* preference to this one, as the toString method may return
* a more user-friendly name.</b> This method is designed primarily for
* use in specialized situations where correctness depends on getting the
* exact name, which will not vary from release to release.
*
* @return the name of this enum constant
*/
public final String name() {
return name;
}
/**
* The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this field. It is designed
* for use by sophisticated enum-based data structures, such as
* {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*/
private final int ordinal;
/**
* Returns the ordinal of this enumeration constant (its position
* in its enum declaration, where the initial constant is assigned
* an ordinal of zero).
*
* Most programmers will have no use for this method. It is
* designed for use by sophisticated enum-based data structures, such
* as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
*
* @return the ordinal of this enumeration constant
*/
public final int ordinal() {
return ordinal;
}
/**
* Sole constructor. Programmers cannot invoke this constructor.
* It is for use by code emitted by the compiler in response to
* enum type declarations.
*
* @param name - The name of this enum constant, which is the identifier
* used to declare it.
* @param ordinal - The ordinal of this enumeration constant (its position
* in the enum declaration, where the initial constant is assigned
* an ordinal of zero).
*/
protected Enum(String name, int ordinal) {
this.name = name;
this.ordinal = ordinal;
}
/**
* Returns the name of this enum constant, as contained in the
* declaration. This method may be overridden, though it typically
* isn't necessary or desirable. An enum type should override this
* method when a more "programmer-friendly" string form exists.
*
* @return the name of this enum constant
*/
public String toString() {
return name;
}
/**
* Returns true if the specified object is equal to this
* enum constant.
*
* @param other the object to be compared for equality with this object.
* @return true if the specified object is equal to this
* enum constant.
*/
public final boolean equals(Object other) {
return this==other;
}
/**
* Returns a hash code for this enum constant.
*
* @return a hash code for this enum constant.
*/
public final int hashCode() {
return super.hashCode();
}
/**
* Throws CloneNotSupportedException. This guarantees that enums
* are never cloned, which is necessary to preserve their "singleton"
* status.
*
* @return (never returns)
*/
protected final Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}
/**
* Compares this enum with the specified object for order. Returns a
* negative integer, zero, or a positive integer as this object is less
* than, equal to, or greater than the specified object.
*
* Enum constants are only comparable to other enum constants of the
* same enum type. The natural order implemented by this
* method is the order in which the constants are declared.
*/
public final int compareTo(E o) {
Enum<?> other = (Enum<?>)o;
Enum<E> self = this;
if (self.getClass() != other.getClass() && // optimization
self.getDeclaringClass() != other.getDeclaringClass())
throw new ClassCastException();
return self.ordinal - other.ordinal;
}
/**
* Returns the Class object corresponding to this enum constant's
* enum type. Two enum constants e1 and e2 are of the
* same enum type if and only if
* e1.getDeclaringClass() == e2.getDeclaringClass().
* (The value returned by this method may differ from the one returned
* by the {@link Object#getClass} method for enum constants with
* constant-specific class bodies.)
*
* @return the Class object corresponding to this enum constant's
* enum type
*/
@SuppressWarnings("unchecked")
public final Class<E> getDeclaringClass() {
Class<?> clazz = getClass();
Class<?> zuper = clazz.getSuperclass();
return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
}
/**
* Returns the enum constant of the specified enum type with the
* specified name. The name must match exactly an identifier used
* to declare an enum constant in this type. (Extraneous whitespace
* characters are not permitted.)
*
* <p>Note that for a particular enum type {@code T}, the
* implicitly declared {@code public static T valueOf(String)}
* method on that enum may be used instead of this method to map
* from a name to the corresponding enum constant. All the
* constants of an enum type can be obtained by calling the
* implicit {@code public static T[] values()} method of that
* type.
*
* @param <T> The enum type whose constant is to be returned
* @param enumType the {@code Class} object of the enum type from which
* to return a constant
* @param name the name of the constant to return
* @return the enum constant of the specified enum type with the
* specified name
* @throws IllegalArgumentException if the specified enum type has
* no constant with the specified name, or the specified
* class object does not represent an enum type
* @throws NullPointerException if {@code enumType} or {@code name}
* is null
* @since 1.5
*/
public static <T extends Enum<T>> T valueOf(Class<T> enumType,
String name) {
T result = enumType.enumConstantDirectory().get(name);
if (result != null)
return result;
if (name == null)
throw new NullPointerException("Name is null");
throw new IllegalArgumentException(
"No enum constant " + enumType.getCanonicalName() + "." + name);
}
/**
* enum classes cannot have finalize methods.
*/
protected final void finalize() { }
/**
* prevent default deserialization
*/
private void readObject(ObjectInputStream in) throws IOException,
ClassNotFoundException {
throw new InvalidObjectException("can't deserialize enum");
}
private void readObjectNoData() throws ObjectStreamException {
throw new InvalidObjectException("can't deserialize enum");
}
}
参考
- 丁振凡编著,《Java 语言程序设计(第2版)》华东交大版,2014.9
- Java 编程的逻辑-微信读书
https://weread.qq.com/web/reader/b51320f05e159eb51b29226kc81322c012c81e728d9d180
